Orthogonal Frequency Division Multiple Access (OFDMA) systems are prevalent today. Typically, in an OFDMA system, the signals of several different users (i.e., entities that wish to communicate over the communication system) will each be assigned one or more unique subcarriers. Each subcarrier is generated and transmitted in a manner that allows all of the subcarriers to be transmitted concurrently without interfering with one another. Therefore, independent information streams can be modulated onto each subcarrier whereby each such subcarrier can carry independent information from a transmitter to one or more receivers.
In one current OFDMA system described in the Multimedia over Coax Alliance (MoCA) industry standard, MoCA 2.0 network coordinators (NCs) (sometimes referred to as network controllers) coordinate synchronous OFDMA transmissions for upstream reservation requests. That is, each participating/requesting network node is scheduled to simultaneously transmit a preamble of a respective message, followed by a payload of the corresponding message that is transmitted simultaneously. Each node transmits on its own set of subcarriers, with a set of subcarriers defining a logical subchannel.
MoCA may be implemented in the context of a hybrid fiber-coaxial (HFC) broadband network 150 shown in FIG. 1A. HFC network 150 includes a fiber optic network 151 and a coaxial network 152. The fiber optic network 151 includes a transport ring 153, including a head-end node 154, one or more distribution hubs 156 (shown as hubs 156a and 156b in this example), and a cable modem termination system (CMTS) 158 for connection to one or more fiber optic nodes 160 (shown as fiber optic nodes 160a and 160b in this example; sometimes referred to as optical nodes) that provide an interface between the fiber portion 151 and coaxial portion 152 of the network 150. Each optical node 160 includes a receiver 161 and a transmitter 162 that provide communications in downstream and upstream directions, respectively, as denoted by arrows 163 and 164. The downstream direction typically corresponds to point-to-multipoint (i.e., broadcast) communications, and the upstream direction typically corresponds to multipoint-to-point communications. Receiver 162a converts the downstream-directed optically modulated signal originating from the transport ring 153 to an electrical signal for distribution to customers 180 (e.g., homes 180) in a network 170, which may be a residential network. A trunk RF amplifier 172 and one or more line RF amplifiers 174 may be used to increase signal strength. Transmitter 161a provides upstream communications from customers 180 to the head-end 154 on a return path.
Referring to FIG. 1B, in a known OFDMA transmission technique, time-frequency slots that are two-dimensional intervals in time-frequency space are granted to transmitters T1 and T2, respectively, which may correspond to respective nodes of residential network 170. T1 is granted a first set of one or more logical subchannels 110a, and T2 is granted a second set of one or more logical subchannels 110b, with T1 granted more bandwidth in this example. Time intervals are granted on the basis of fixed time duration, which may correspond to a given number of symbols (e.g., 20 symbols). Two time intervals 120a and 120b of equal duration are shown in this example. The transmission schedule is synchronous in this example, and the time-frequency slots conform to a periodic grid.
Each data stream (e.g., packet) that is sent starts transmission at the same time so that the preambles of packets sent by respective transmitters are aligned in time. In this example, packets 132 and 142 are sent at the same time (i.e., at the start of time interval 120a) so that their respective preambles 133 and 143 are aligned in time. However, packets may have different lengths, e.g., due to differing lengths of respective payloads 134 and 144. Therefore, if a shorter packet (e.g., packet 132) is sent on one set of subchannels (e.g., subchannels 110a), and a longer packet (e.g., packet 142) is sent on another set of subchannels (e.g., subchannels 110b), the subchannels on which the shorter packet was sent will be padded or idle, waiting for the completion of the transmission of the longer packet, as shown by idle interval 122. Additional packets may be sent in the next time interval 120b. 
In particular, in a network where all upstream traffic is destined for a network coordinator (NC), the beginning and end of various packet transmissions may not align precisely. This misalignment may be due by different nodes transmitting packets of various lengths (e.g., from 64 to 1518 bytes each). Alternatively, this misalignment may be due to different nodes transmitting over separate subchannels with differing bit loadings and subchannel widths. For example, a first node may be required to transmit its packets over subchannels corresponding to narrower bandwidth than a second node. The first node may use a lower-order bit loading than the second node in order to improve the fidelity of the transmission. Since the system is constrained to synchronous OFDMA, a node with a short packet (destined for the NC) might have to wait for another node to finish transmitting a long packet (also destined for the NC) before the two nodes could synchronously transmit respective their preambles and new payloads, limiting flexibility and efficiency.